Gonadotropin-releasing hormone (GnRH) regulates gonadotropin release from the anterior pituitary via a specific receptor-mediated event. In addition to innervating the median eminence, GnRH-containing neurons project to various extrahypothalamic structures, suggestng that GnRH may have additional actions in extrapituitary tissues. However, specific GnRH receptors have not been identified in the brain, possibly due to the low concentration of receptors within discrete brain regions. The PI's laboratory has synthesized a luciferin-GnRH derivative for use as a probe to study specific GnRH binding sites. Because of the sensitivity of the luciferin-luciferase reaction, the bioluminescent ligand is 10-50 fold more sensitive than 125I-GnRH analogs, and permits us to measure extremely low levels of GnRH receptors in isolated brain regions. Using this technique, we have identified specific GnRH receptors in rat brain. The objectives of this proposal are to localize and characterize specific GnRH receptors in the rat brain and study the regulation of these receptors during varied reproductive states. The increased sensitivity of the GnRH-luciferin probe will enable us to examine changes in GnRH receptor levels in various brain regions during pubertal development, the estrous cycle, pregnancy, lactation and aging in female rats as well as in male rats of different ages. The role of steroid hormones on brain GnRH receptor levels will be studied in castrated males and females with and without steroid hormone replacement. Further studies will be directed at localization and characterization of GnRH receptors in human brain regions. The results of these experiments will provide important information concerning the physiological function of GnRH in the brain.